1. Field of the Invention
The present invention relates to a multicarrier transmission apparatus and a multicarrier transmission method for use with an x Digital Subscriber Line (xDSL; x represents A, S, V, and the like) to conduct data transmission through a metallic cable such as a telephone line at a high transmission rate of several megabits per second, and in particular, to a multicarrier transmission apparatus and a multicarrier transmission method to conduct data transmission at a high transmission rate in an environment in which noise occurs abruptly.
2. Description of the Prior Art
Recently, attention has been drawn to an xDSL technique capable of accomplishing data transmission at a high transmission rate of several megabits per second using a metallic cable such as a telephone line. Especially, an Asymmetric Digital Subscriber Line (ADSL) has collected attention. On the ADSL, a forward or upstream line and a reverse or downstream line adopt mutually different transmission rates. The asymmetric characteristic is suitable for the access to the Internet.
Referring now to FIG. 1, description will be given of a system configuration of a general ADSL transmission system.
As can be seen from FIG. 1, the ADSL transmission system includes ADSL subscriber equipment 100, a subscriber telephone 101, a splitter 102 on the subscriber side, ADSL office equipment 104, and a splitter 106 on the office side.
The ADSL subscriber equipment 100 is connected via the splitter 102 on the subscriber side to a line 103. The subscriber telephone 101 is linked via the splitter 102 on the subscriber side to the line 103.
The ADSL office equipment 104 is connected via the splitter 106 on the office side to the line 103. The exchange 105 is linked via the splitter 106 on the office side to the line 103.
The splitters 102 and 106 are used to split signals on the line 103 into telephone signals and data signals for ADSL.
The splitter 102 on the subscriber side is coupled with the subscriber telephone 101 side when the signal on the line 103 is a telephone signal and with the ADSL subscriber equipment 100 when the signal is an ADSL data signal.
The splitter 106 on the office side is connected to the exchange 105 side when the signal on the line 103 is a telephone signal and with the ADSL office equipment 104 when the signal is an ADSL data signal.
The ADSL office equipment 104 includes a Digital Subscriber Line Multiplexer (DSLAM). The equipment 104 is coupled via the DSLAM and a provider with the Internet. The multiplexer converts data transmitted in the form of analog signals into digital signals to feed the resultant signals to the provider.
The ADSL transmission system converts a digital signal into an analog signal through a modulation and demodulation scheme called a Discrete Multi-Tone (DMT) scheme to achieve high-speed data transmission at a high transmission rate.
In the DMT system, a transmission side conducts Quadrature Amplitude/Phase Modulation (QAM) for 256 carriers and multiplexes the modulated carriers through an inverse Fourier transform to deliver the multiplexed signals to a reception side. When the signals are received, the reception side extracts the carriers from the signals using a Fourier transform to demodulate the extracted carriers.
In an ADSL transmission system, when a line of the ADSL system and a line of an Integrated Service Digital Network (ISDN) are configured in one bundle of cables, the line of the ADSL system is affected by the ISDN line. This possibly leads to a problem of occurrence of noise that lowers the data transmission rate on the line of the ADSL system. Among the influences from the ISDN line onto the line of the ADSL system, crosstalk noise from the ISDN line is most troublesome.
To suppress such influence of the ISDN, it is also possible in the ADSL transmission system to separately accommodate the line of the ADSL system and the ISDN line in different cable bundles. However, in the ADSL transmission system of this configuration, there arises another problem that the load imposed on the operator increases. In this situation, for the ADSL transmission system using a cable bundle including both of the lines of the ISDN and ADSL systems, there has been desired a transmission method to prevent the reduction in the data transmission rate.
Referring now to FIG. 2, description will be given of crosstalk noise taking place on the line of ADSL system when the ISDN line of the TCM scheme is employed. FIG. 2 shows crosstalk noise appearing on an ADSL Transceiver Unit-Remote side (ATU-R), which is a device on a terminal side of the line of ADSL system, due to data transmission through the TCM-ISDN line while reverse or downstream data transmission is taking place. On the TCM-ISDN line, data is alternately transmitted in the upstream and downstream directions every 1.25 milliseconds (ms).
During downstream data transmission on the line of ADSL system, when data is transmitted in the upstream direction on the TCM-ISDN line, a high-power signal before attenuation thereof on the TCM-ISDN line influences an attenuated signal on the line of ADSL system. This disadvantageously causes a Near End Cross Talk (NEXT) in the ATU-R which is the terminal device of the ADSL system.
Also, during a period of downstream data transmission on the line of ADSL system, when data is transmitted in the downstream direction through the TCM-ISDN line, a signal on the TCM-ISDN line affects an attenuated signal on the line of ADSL system. This results in a Far End Cross Talk (FEXT) in the ATU-R which is a terminal of the line of ADSL system. In this regard, a similar phenomenon occurs also in an ADSL Transceiver Unit-Center Side (ATU-C) which is a device on the central office side of the ADSL communication system.
Next, description will be given of a quantity of the crosstalk noise by referring to FIG. 3. FIG. 3 shows quantities of the crosstalk noise. As shown in FIG. 3, a noise quantity at occurrence of “NEXT” is more than a noise quantity at occurrence of “FEXT”. This is because a high-power signal not attenuated on the TCM-ISDN line affects a signal attenuated on the line of ADSL system. Paying attention to the difference between the noise quantities, there has been proposed a method in which data is transmitted by changing an amount of transmission data between NEXT and FEXT. In this method called a dual bit map method, at occurrence of FEXT in which the noise quantity is less than a predetermined threshold value, a larger amount of data is transmitted as shown in FIG. 3. At occurrence of NEXT in which the noise quantity is more than a predetermined threshold value, a smaller amount of data is transmitted.
Since the quantity of noise periodically changes in an ADSL transmission system in which a TCM-ISDN line is adjacent to a line of ADSL system, it is a common practice that a Signal To Noise Ratio (SNR) is measured for carriers of the upstream and downstream directions to obtain a bit distribution ratio according to the measured SNR values.
Referring next to FIG. 4, description will be given of a conventional ADSL transmission system.
Configuration on ATU-C 300 Side
Description will be given of a system configuration on the ATU-C 300 side.
The ATU-C 300 side includes in its transmission section a Cyclic Redundancy Check (CRC) error processing unit 315 to add a CRC code to data sent from an upper-level system, a scramble processing and error correction (scram & Forward Error Correction (FEC)) unit 301 which executes scramble processing for the data including the CRC code and which adds an error correction code of the Reed-Solomon system to the resultant data, a mapping unit 302 which changes a transmission power distribution ratio and a bit distribution ratio of each carrier according to timing at which a noise level alters to thereby add the bit distribution ratio and the transmission power distribution ratio to the carrier, an inverse Fourier transform unit 303 which modulates and multiplexes a multivalue Quadrature Amplitude Modulation (QAM) signal produced from the mapping unit 302, and a digital-analog converter unit 304 to convert an output signal from the inverse Fourier transform unit 303 into an analog signal to transmit the signal as a downstream analog signal to the reception side.
The ATU-C 300 includes in a reception section an analog-digital converter unit 305 to convert an analog signal sent from the ATU-R 400 into a digital signal, a Fourier transform unit 306 to conduct a Fourier transform for the digital signal, a demapping unit 307 to change a bit distribution ratio and a transmission power distribution ratio according to timing at which a noise level varies to demodulate the signal transmitted thereto, a scramble processing and error correction (scram & FEC) unit 308 to execute scramble processing for the data and conduct an error correction for the data to thereby restore correct data, and a CRC error detector unit 314 to execute processing by use of a predetermined expression to check the CRC code added to the data and detect a CRC error.
The ATU-C 300 further includes a pseudo-random signal generator unit 310, a noise tone generator unit 311, and a bit-power distribution ratio calculating unit 312. FIG. 5 shows a configuration of the calculating unit 312 in detail.
Configuration on ATU-R 400 Side
Description will next be given of the configuration on the side of the ATU-R.
The ATU-R 400 includes in a transmission section thereof a CRC error processing unit 415 to add a CRC code to data sent from an upper-level system, a scramble processing and error correction (scram & FEC) unit 401 which executes scramble processing for the data including the CRC code and which adds an error correction code of the Reed-Solomon system to the obtained data, a mapping unit 402 to change a transmission power distribution ratio and a bit distribution ratio of each carrier according to timing at which a noise level alters to thereby add the bit distribution ratio and the transmission power distribution ratio to the carrier, an inverse Fourier transform unit 403 which modulates and multiplexes a multivalue QAM signal produced from the mapping unit 402, and a digital-analog converter unit 404 which converts an output signal from the inverse Fourier transform unit 403 into an analog signal to transmit the signal as an upstream analog signal to the transmission side.
The ATU-C 400 includes in a reception section an analog-digital converter unit 408 to convert an analog signal sent from the ATU-C 300 into a digital signal, a Fourier transform unit 407 to conduct a Fourier transform for the digital signal, a demapping unit 406 to change a bit distribution ratio and a transmission power distribution ratio according to timing at which a noise level varies to demodulate the signal transmitted thereto, a scramble processing and error correction (scram & FEC) unit 405 which executes scramble processing for the data and conduct an error correction for the data to thereby restore correct data, and a CRC error detector unit 414 which executes processing by use of a predetermined expression to check the CRC code added to the data and detect a CRC error.
The ATU-R 400 additionally includes a pseudo-random signal generator unit 409 and a bit-power distribution ratio calculating unit 410. FIG. 6 shows a configuration of the calculating unit 410 in detail.
In the ADSL transmission system of FIG. 4, during data transmission in the ISDN downstream direction, NEXT occurs in the ATC-C 300 and FEXT takes place in the ATC-R 400. During data transmission in the ISDN upstream direction, FEXT occurs in the ATC-C 300 and NEXT takes place in the ATC-R 400.
To secure a required data transmission capacity under a noisy environment, the pseudo-random signal generator (310, 409) generates pseudo-random signals by sequentially assigning data in the form of a predetermined pseudo-random sequence to each carrier used for data transmission. The resultant pseudo-random signal is fed to the inverse Fourier transform unit (303, 403) to be delivered via the digital-analog converter (304, 404) to the communicating station side.
The bit-power distribution ratio calculation unit (312, 410) obtains, by use of the pseudo-random signal created by the pseudo-random signal generator (409, 310) on the communicating station side, a bit distribution ratio and a transmission power distribution ratio which are assigned to each carrier for data transmission under NEXT and FEXT. The calculation unit (312, 410) then stores the bit distribution ratio and the transmission power distribution ratio attained under both NEXT and FEXT in the demapping unit (307, 406) on the own station side and the mapping unit (302, 402) on the communicating station side.
Description will now be given of operation of the bit-power distribution ratio calculation unit (312, 410) to obtain a bit distribution ratio and a transmission power distribution ratio. Since the ATU-C 300 and the ATU-R 400 conduct substantially the same operation, description will be given of only the processing to attain a bit distribution ratio and a transmission power distribution ratio in the downstream direction.
During a training period to calculate a bit distribution ratio and a transmission power distribution ratio which are assigned to each carrier, the pseudo-random signal generator 310 modulates amplitude of each carrier used for data transmission into amplitude associated with a string of bits of predetermined data assigned in association with a predetermined pseudo-random sequence. The signal generator 310 delivers the modulated amplitude of each carrier to the inverse Fourier transform unit 303.
The Fourier transform unit 303 conducts the Fourier transform for each carrier having the modulated amplitude to produce a voltage value in a digital format by amalgamating the carriers. The digital-analog converter 304 converts a digital voltage value into an analog signal having an actual voltage value to send the signal to a line.
The ATU-R 400 converts by the analog-digital converter 408 the analog signal from the ATU-C 300 into a digital voltage value. The Fourier transform unit 407 conducts the Fourier transform for the digital voltage value to obtain each carrier with modulated amplitude and delivers the carrier to the bit-power distribution ratio calculation unit 410.
The calculation unit 410 calculates, by a downstream SNR evaluation unit, SNR values of each carrier under NEXT and FEXT to obtain a mean SNR value of each carrier.
In FIG. 7, “A” indicates an SNR mean value for occurrence of FEXT and an SNR mean value for occurrence of NEXT evaluated by the downstream SNR evaluation unit.
The downstream SNR evaluation unit shown in FIG. 6 keeps in “NEXT SNR” the SNR mean value under NEXT and in “FEXT SNR” the SNR mean value under FEXT.
The bit-power distribution ratio calculation unit 410 calculates a bit distribution ratio and a transmission power distribution ratio of each carrier for each noise level according to the measured SNR mean value of each carrier and feeds the distribution ratios to the demapping unit 406 to store the ratios therein and then delivers the ratios to the mapping unit 402. In FIG. 7, “B” conceptually indicates operation to determine the bit distribution ratio of each carrier according to the SNR mean value evaluated by the downstream SNR evaluation unit.
During the training period to calculate a bit distribution ratio to be assigned to a carrier for data transmission and a transmission power distribution ratio to be used for the carrier, the mapping module 402 assigns to a predetermined carrier a predetermined number of bits of the information of the bit distribution ratio and the transmission power distribution ratio calculated by the calculation module 410 to deliver the resultant carrier to the inverse Fourier transform module 403.
The inverse Fourier transform module 403 conducts the inverse Fourier transform for the predetermined carrier from the mapping module 402 to produce a voltage value represented in a digital format. The digital-analog converter 404 converts the digital voltage value into an analog signal of the voltage value to feed the signal to the line.
The ATU-C 300 converts by the analog-digital converter 305 the analog signal from the ATU-R 400 into a voltage value expressed in a digital format. The Fourier transform module 306 conducts the Fourier transform for the digital voltage value to attain each carrier with modulated amplitude.
The demapping module 307 acquires information of the bit and transmission power distribution ratios from the predetermined carriers assigned with predetermined numbers of bits and sends the information to the mapping module 302 to store the information therein.
The mapping module (302, 402) selects, form the two kinds of ratios, i.e., the bit and transmission power distribution ratios calculated through the above processing, a bit distribution ratio and a transmission power distribution ratio according to the noise level at data transmission and adds the bit distribution ratio and the transmission power distribution ratio to each carrier. The demapping module (307, 406) obtains, by use of a bit distribution ratio and a transmission power distribution ratio equal to those selected according to the noise level in the communicating station, data assigned to the carrier.
The ADSL transmission system shown in FIG. 4 includes a noise sync tone generator 311 on the ATU-C 300 side and a clock detector 411 and a bit-power distribution ratio selector 412 on the ATU-R 400 side.
It is assumed that the clock signal on the ATU-C 300 side is synchronized with timing at which the noise level changes and the noise level change timing is known. When noise is, for example, crosstalk from the TCM-ISDN line, NEXT and FEXT alternately take place every 1.25 ms, and hence the SNR of each carrier also changes every 1.25 ms. Therefore, it is required that the transmission section of the ATU-C 300 receives a clock signal of which amplitude changes every 1.25 ms synchronized with the timing of the noise level change and then delivers the clock to the reception section of the ATU-R 400. For this purpose, the noise sync tone generator 311 produces a noise sync tone signal of which a signal level alters at timing synchronized with the clock signal and feeds the signal to the ATU-R 400. More specifically, according to the clock signal synchronized with timing of the noise level change, the generator 311 alters amplitude of a predetermined carrier in synchronization with the noise level change timing.
The clock detector 411 detects timing of change in the noise level according to the change in the carrier amplitude obtained by the Fourier transform module 407 and sends the noise level change timing to the bit-power distribution ratio selector 412.
The selector 412 recognizes the timing of the noise level change using the notification from the clock detector 411 and designates, by using the bit and transmission power distribution ratios stored in the mapping module 402, a bit distribution ratio and a transmission power distribution ratio that is adopted in data transmission according to the noise level.
Using the bit and transmission power distribution ratios stored in the demapping module 406, the bit-power distribution ratio selector 412 specifies a bit distribution ratio and a transmission power distribution ratio equal respectively to those employed by the ATU-C 300 according to the noise level, the specified bit and transmission power distribution ratios being used for data demodulation.
FIG. 8 shows a configuration of a hyperframe including 345 symbols. In FIG. 8, symbols on the left side of a dotted line A are associated with a little crosstalk noise from the ISDN line (FEXT). For the symbols, a large number of bits can be allocated to the carrier. Symbols interposed between the dotted line A and a dotted line B are associated with much crosstalk noise from the ISDN line (FEXT). For the symbols, only a few bits can be allocated to the carrier.
When data transmission is stated at symbol 0 in synchronization with timing of occurrence of FEXT from the ISDN line, timing to receive symbol 344, i.e. the 345th symbol synchronizes with timing of change in the crosstalk noise from the ISDN line. It is therefore possible to conduct symbol transmission beginning at the 346th symbol at timing synchronized with the timing of occurrence of FEXT from the ISDN line as shown in FIG. 8. The bit-power distribution ratio selector 412 stores, for each sequential symbol transmission, a bit distribution ratio and a transmission power distribution ratio selected from the bit distribution and transmission power distribution ratios.
The inverse Fourier transform module 303 receives signals from the pseudo random signal generator 310, the noise sync tone generator 311, and the mapping module 302. However, these signals are not delivered to the transform module 303 at the same time. That is, the module 303 conducts the inverse Fourier transform for the signals received at mutually different points of time to deliver resultant signals to the digital-analog converter 304. The modules described above are controlled by a sequencer (not shown). Under control of the sequencer, the generators 310 and 311 send signals to the inverse Fourier transform module 303. The module 303 beforehand recognizes the sequence in which the above modules deliver the signals under control of the sequencer.
In association with the crosstalk noise from the TCM-ISDN on the adjacent line, FEXT and NEXT alternately occur every 400 hertz (Hz) and the noise period is synchronized with 400 Hz as shown in FIG. 3. Therefore, in the conventional ADSL transmission system, the period of crosstalk noise from the TCM-ISDN is predicted using a 400 Hz clock signal to thereby prevent errors due to periodically occurring noise.
However, there exists a problem that noise like “burst” takes place for a short period of time during communication to resultantly cause disconnection of the line connection. In the ADSL transmission system of the prior art, when such burst-like noise takes place for a short period of time during communication, it is not possible to predict a Power Spectrum Density (PSD) and a period of the noise. The PSD cannot be fully measured during the ordinary initialization and training phase and hence the bit distribution ratios used for the data transmission are not appropriate. Therefore, at occurrence of non-periodic noise, the multicarrier transmission cannot be efficiently conducted.
In a technical article published preceding the present invention, for example, Japanese Patent Reference No. 3348719, there is described a technique in which a transmission power distribution ratio of each carrier of the multicarrier is calculated according to a period of periodically changing noise. According to the distribution ratio, data is transmitted such that the multicarrier transmission is efficiently accomplished under periodically changing noise.
Another article, for example, Japanese Patent Reference No. 3319422 described a technique in which data transmission is achieved utilizing a multicarrier between first and second communication stations in a noisy environment where timing of the noise level change is known.
In accordance with the technique described in Japanese Patent Reference No. 3348719, the multicarrier transmission is efficiently carried out under the periodically changing noise. The technique of Japanese Patent Reference No. 3319422 is multicarrier transmission in a noisy environment where the noise level change timing is known. In the techniques of these articles, consideration has not been given to countermeasures to deal with a situation at which non-periodic noise occurs.